As a web dev I made two browser games with no framework (for one I used the Matter.js physics engine) and it was a good experience. The browser has great built in apis like the Canvas element so it isn't completely "from scratch", but you'll still get a great sense for the basics of making a game loop etc from the ground up and you can use your previous experience.

I've also played around with Unity a bit but haven't released anything or finished any project. If I wanted to transition to full time game dev I would probably start learning Unreal or Godot as an engine for major projects... or focus on making browser games, maybe with a JS game engine like Melon or Phaser, or continue to roll my own JS games. The big engines like Unity can compile to web but it's a subpar experience and I wouldn't use it if my main target was in browser.

Web devs like us will have some advantages in already being exposed to some of the networking and stuff related to multiplayer, auth, leaderboards, etc.